A row has blown up between several northern US states about how to block an especially voracious species of non-native fish from entering the Great Lakes and potentially devastating their multi-billion-dollar fishing industry.
Some experts believe that the Asian carp, which can grow up to four feet long and weigh as much as seven stone, may already have breached electrified barriers placed seven years ago in the waterways that connect the Mississippi River to Lake Michigan close to Chicago.
That would mean that the fish may be just a few miles off the lakes system, if they are not already there. Once in, they could never be expelled and their rapacious consumption of algae could wipe out other species and close down the lakes' annual $7bn (£4.5bn) fishing and boating industries.
In freezing temperatures, teams of fishermen and specialists from the Illinois Department of Wildlife and Fisheries began scouring canals around Chicago last week for the fish. DNA evidence has been detected in waters on the wrong side of the barriers.
Earlier this month, the Obama administration released $78m to help control the carp on the Windy City's doorstep.
Illinois is on the defensive as Michigan has filed a lawsuit with the US Supreme Court, supported by five other states, demanding that it immediately close the locks that allow ships to navigate to and from the Mississippi.
Illinois has argued that closing the Chicago Sanitary and Ship Canal is not tenable because of the enormous damage it would do to its economy. Cargo worth around $16bn is shipped through the waterway each year. But Michigan has released a report saying that Illinois's fears are overblown. "The claims that even a temporary closure will devastate the local economy and Illinois's role in the regional, national and global economy cannot reasonably be supported," it said.
Tom Marks, a charter boat captain on Lake Ontario, spoke of his fears at a public meeting in Michigan on Wednesday. "We lose the Great Lakes to the Asian carp, you're not going to get them back."